"Main scale" control on a material from Poliigon Material converter
The Add-on is responsible for adding this field there. But it only gets drawn if a special property, managed by the add-on, is present in the material properties. These custom properties don't get copied over when you duplicate a material.
Basically this is just a shortcut to the scale values of a mapping node inside the material. I think it is a bit convoluted for what it does, but you should contact the support if you want to make them aware of this limitation.
Digging into the add-on files I found this:
if mat and mat.pmc_matprops.workflow != '':

        # get the mapping nodes, if found
        main_map = None
        for n in mat.node_tree.nodes:
            if 'main_map' in n: # ie n['main_map'] exists
                main_map = n

        if main_map:
            row = col.row(align=True)
            if hasattr(main_map, &quot;scale&quot;):
                row.prop(main_map,&quot;scale&quot;, text=&quot;Main scale&quot;)
            elif 'Scale' in main_map.inputs:
                row.prop(main_map.inputs['Scale'], &quot;default_value&quot;, text=&quot;Main scale&quot;)

Explanation :
On the first line the method checks if the material has a custom property named pmc_matprops. I believe it is added by the add-on when first instantiating the material. When you duplicate the material, the property doesn't get copied over.